I'm aware of quite a few people who use the 1000HX for packet
  radio - as the 1000EX is a very similar computer, the
  differences should be small. Unfortunately, I don't have their
  names/addresses handy :-/ You can purchase a serial board for
  the EX from: DCS Industries *or PC Enterprises as well as from
  your local Radio Shack. I believe its approximately the same
  price for all of them. I will e-mail the Tandy 1000 FAQ and you
  can find the addresses for DCS Industries and PC Enterprises in
  it.